`router/dataplane.go`: information hiding and refactoring

[jcp19]

Improvements:

• drop isComparable from initialization spec
• drop key field and have a predicate WithKey(key) instead (or a second predicate altogether that reveals the state of the router, while still requiring the dataplaneMutexInvariant to modify the dataplane)
  • instead, implement it as a ghost field
• drop all wildcards wherever possible (specially in predicates and method spec)
• use const perm instead of hardcoded permissions everywhere
• Bring changes from PR refactor parts of the dataplane #122